The present invention relates generally to methods for inhibiting corrosion of metal-containing surfaces that are exposed to aqueous solutions for short or prolonged periods and, in particular, relates to processes for testing tanks, containers, and other structures in which water is used as a test medium that is brought into contact with one or more surfaces of such structures. The present invention relates also to methods for inhibiting corrosion of metal surfaces exposed to normal atmospheric conditions, including environments in which humidity may not be controlled.
Various structures having aluminum or other metallic surfaces may be subject to testing using water or water-containing solutions or materials. Such testing generally involves filling a structure to be tested with water or an aqueous solution, subjecting the structure to various stresses, and then examining the walls of the structure, and in particular, any seals or joints in the walls, for deformation, leakage of water, or other irregularities. When such testing is conducted using source water or a similar water-containing solution over short periods (e.g., of less than four hours), minor discoloring corrosion of aluminum-containing surfaces and components of the structure may occur. When testing is conducted using source water or a similar water-containing solution over extended periods (e.g., of more than four hours), pitting corrosion and other substantial degradation of metal-containing surfaces and components may occur. This, in turn, can diminish the integrity of the structure being tested.
Space launch vehicles are generally constructed of components comprised of lightweight aluminum alloys. For example, the interior walls of booster tanks used in propulsion, and components within the booster tanks, are often fabricated from aluminum alloy materials. To evaluate the strength and integrity of such booster tanks, it is desirable to conduct structural testing, such as static load or proof pressure hydrostatic testing. This testing generally involves filling the booster tanks with a test medium, such as water, for periods varying from one to seven weeks, or more. However, the use of source water or a similar aqueous solution as a test medium, typically results in substantial corrosion and degradation of the interior aluminum alloy walls and components of the booster tank. Such corrosion may lead to undesirable deposits of materials on the walls and components of the booster tank. Such deposits are incompatible with liquid oxygen and rocket propellant fuels that may fill the booster tank, can be a fire hazard, and may impair the performance of working parts exposed to these propellants such as the rocket propellant engine. The interior walls of such booster tanks may be complex structures, having ridges, such as may be found in isogrid structures, and other test components that extend radially inwardly from the wall, into the interior cavity of the booster tank, making difficult the application of paint or other conversion coatings on such walls to protect them from corrosion.
A further concern for space launch vehicles is that such structures may be produced many months before projected use. Such vehicles may have to be transported over long distances from the point of production to the point of use. During transportation and storage, it may not always be possible to maintain such vehicles in a controlled environment, and they may be exposed to atmospheric moisture than can cause corrosion of aluminum-containing surfaces. It is therefore desirable to protect such aluminum-containing surfaces from corrosion during fluid testing and also later during transportation and storage, prior to use.
An object of the present invention is to provide a method for protecting the aluminum alloy and other metal surfaces of tanks, containers, and other structures from corrosion that may occur when such surfaces are exposed to an aqueous media (e.g., during testing, such as performance or validation testing).
A further object of the present invention is to provide a method for inhibiting corrosion of metal-containing surfaces that may be exposed to an aqueous medium or normal atmospheric conditions such as a humid environment.
The present invention relates to a system and method for inhibiting corrosion of metal-containing surfaces that may occur when such surfaces are exposed to water or an aqueous solution, such as during testing processes. More specifically, the invention is directed to a method for inhibiting corrosion through the use of an alkali metal silicate as an additive to the water or aqueous solution that will be brought into fluid contact with a metal-containing surface.
In one aspect of the invention, the method may include the steps of bringing water combined with an alkali metal silicate into contact with an aluminum-containing surface and maintaining such contact between the mixture of water and silicate and the aluminum-containing surface for at least a first period of time. Contacting the mixture of water and silicate with such a surface results in the formation of a thin silicate film on the surface that protects the aluminum-containing surface from corrosion. The addition of this mixture of silicate and water, in appropriate concentrations, thus functions to inhibit the corrosion that ordinarily results from exposure of such a surface to source water or a solution containing source water. In one embodiment, the alkali metal silicate comprises sodium silicate. The same effect may be achieved where the surface comprises stainless steel, alloy/carbon steel, and other metallic materials having low resistance to corrosion.
In another aspect of the present invention, the method includes the steps of forming a test medium comprised of an alkali metal silicate and water, and contacting the silicate and water test medium with an aluminum-containing surface to form a protective film on the surface during the testing process. The step of forming the combination of alkali metal silicate and water may comprise the step of mixing the silicate with water, in appropriate concentrations, to form a silicate solution, preliminary to the step of contacting the solution with the aluminum-containing surface. The step of forming the combination of alkali metal silicate and water may alternatively include the step of contacting the aluminum-containing surface of a structure to be tested with water and then mixing silicate with the water to form a silicate solution test medium that in turn forms a protective film on the aluminum surface. In one embodiment of the invention, the silicate comprises sodium silicate.
For purposes of enhancing the anti-corrosive effect of the invention, the water to be combined with the silicate may comprise deionized water. Deionized water substantially lacks the ionic impurities that can contribute to corrosion of aluminum and other metal surfaces in contact with this water. In one embodiment, the invention includes a preliminary step of deionizing water to be used in the test medium, before the step of forming the test medium.
A further aspect of the method of the present invention is directed to protecting the aluminum-containing inner wall surface of a launch vehicle propellant tank during testing of such tank, where the test medium is comprised of deionized water. In this aspect of the invention the method generally includes the steps of contacting an aqueous test medium comprising a sodium silicate solution with at least a first section of the inner wall surface of the tank, and then, while maintaining contact between the test medium and the first section of the inner wall surface, applying various stresses to the tank as may be required by the test protocol. The sodium silicate solution of the test medium forms a protective film on the section of the inner wall surface in contact with the test medium that inhibits corrosion, so avoiding the severe damage to the walls and components within the tank that may otherwise occur on exposure to test medium containing source water. In one embodiment of the method of the present invention, the method includes forming the test medium by combining sodium silicate with deionized water. This step may include dissolving sodium silicate into the water. The step of forming the test medium may be performed outside of the internal containment are of the propellant tank to be tested. Alternatively, the step of forming the test medium may comprise the steps of introducing the water into the internal containment area of the tank and adding the silicate (e.g., by injection) to the internal containment area during the step of introducing the water. The method may further include the step of deionizing the water to be used in the test medium prior to combining with the silicate.
The present invention is adaptable to systems or structures in which an aqueous solution flows over, sprays, or immerses components, such as an inner wall of a tank or container, comprises metallic material, such as aluminum. It is adaptable for systems in which water or an aqueous solution, or a solid or semi-solid material containing water or stored in water, is contained or held within a container having walls comprising metallic materials such as aluminum, such as tanks, vats, bins, silos, pipes, vats, or sinks. It is also adaptable for systems in which a metal-containing surface is exposed to atmospheric conditions where the humidity level is not controlled.